The invention relates to an image sensor for a scan head usable in document scanners, facsimile machines, photocopiers and the like. In particular, the invention relates to an image sensor in which a waveguide array is used to transmit light from the scanned object to a light sensor array.
Scanners generate an electrical signal in response to light reflected by an object being scanned (the object). The object is usually a piece of paper, but other kinds of object can be scanned. Facsimile machines and certain types of photocopier operate similarly. Such devices include a scan head in which the light reflected by object is transmitted to an array of light sensors that generates the electrical signal. The entire object is scanned by moving the scan head relative to the object or by moving the object relative to the scan head. The scan heads of older types of scanners use reduction optics composed of multiple lenses and mirrors to transmit the light reflected by the object to an array of silicon light sensors. Reduction optics have excellent optical characteristics, but are bulky, delicate, expensive and may require complex alignment during manufacture production. Moreover, in the common arrangement in which the scan head moves relative to the object, the bulk of the reduction optics increases the size of the enclosure in which the scan head moves, further increasing the size, mass and cost of the scanner.
The scan heads of an increasing number of scanners use a lens array based image sensor in which a linear array of gradient index lenses forms a 1:1 image of the object on a linear light sensor array substantially equal in width to the maximum scan width of the scanner. The number of sensor elements per unit length in the light sensor array is equal to the optical resolution of the scanner. For example, the light sensor array in a scanner having a optical resolution of 24 points/mm (600 dpi) has 24 light sensors/mm (600 light sensors/inch).
Many lens array-based scanners obtain color information by sequentially illuminating the object with red, green and blue light. Such light may be generated by a linear array of red, green and blue light-emitting diodes (LEDs), for example, disposed perpendicular to the scan direction of the scan head. Another type of lens array-based scanner scans the object three times. In each scan, the object is illuminated with light from a white light source filtered by a filter of a different color or by light generated by LEDs of a different color. Finally, lens array scanners may employ three light sensor arrays, each equipped with a color filter of a different color. In this case, the object can be scanned once using a white light source. However, the cost of providing three page-width light sensor arrays represents a substantial fraction of the component cost of the scanner.
Scanners that employ a scan head incorporating a lens array-based image sensor are considerably less bulky and weigh less than scanners that employ a scan head incorporating reduction optics but have a number of performance shortcomings. The type of lens array-based scanners that illuminate the object with light of three different colors have a slow scan speed. Some scanners require as long as two minutes to scan a single A4 page. Using the lens array to focus light reflected by the object onto the light sensor array reduces the depth of field of the scanner compared with that of a scanner that incorporates reduction optics. Depth of field values of about 0.33 mm are typical. Such a small depth of field is not usually a problem when the object is a flat piece of paper. However, when the object has depth, such a page of an open book, the image formed on the light sensor array of parts of the object displaced depthwise from the platen on which the object rests will be out of focus. Finally, the 220 mm wide light sensor array used in a typical lens array-based scanner is considerably more expensive than the smaller light sensor arrays used in scanners that incorporate reduction optics. The cost of the light sensor array represents a significant portion of the component cost of the scanner. This is especially so if three page-width light sensor arrays are used to increase the scan speed.
In United States patent application Ser. No. 08/898,935, the disclosure of which is incorporated into this application by reference, the applicant disclosed a scan head that uses a waveguide array to transmit the light reflected by an object to a light sensor array. However, versions of this scan head for use in scanners with an optical resolution of 24 points/mm (600 dpi) or 48 points/mm (1,200 dpi) are larger than comparable lens array-based scan heads.
Consequently, what is needed is a scan head that combines the compactness and low mass of known lens array-based scan heads, but that has a faster scan speed, a greater depth of field, and an optical resolution of at least 12 points/mm.
The invention provides a scan head for scanning an object in a scan direction. The scan head comprises an elongate light source, an elongate light sensor array and a planar waveguide array. The light source is disposed perpendicular to the scan direction. The light sensor array is disposed parallel to the light source. The planar waveguide array includes an input end and an output end, and is curved about an axis parallel to the light source. The input end is located to receive light from the light source reflected by the object. The output end is located adjacent the light sensor array. The waveguide array includes tapered waveguides linearly arrayed in an array direction parallel to the light source. Each of the waveguides has a width in the array direction that decreases towards the output end of the waveguide array.
The scan head may additionally comprise a rod lens disposed parallel to the light source. The rod lens is arranged to form an image of a narrow strip of the object on the input end of the waveguide array.
Each of the waveguides may include a tandem arrangement of tapered waveguide segments. Each of the waveguide segments has a width that progressively decreases towards the output end of the waveguide array.
The waveguide array may additionally include a layer of absorbent material adjacent a surface of the waveguide array that is convexly curved when the waveguide array is curved about the axis.
The invention also provides a waveguide array that comprises tapered waveguides linearly arrayed in an array direction. Each of the waveguides has a width that decreases between an input end and an output end, and includes a tandem arrangement of tapered waveguide segments. Each of the waveguide segments has a width that progressively decreases towards the output end of the waveguide.